1. Field of the Invention
The present invention relates to an electrode material that can be advantageously used as a fuel electrode in a fuel cell, and a fuel cell battery using a fuel electrode formed from such an electrode material. The fuel cell battery of the present invention not only can achieve higher fuel electrode performance than a conventional fuel cell battery using a nickel cermet as a fuel electrode, but also can effectively generate electricity without pre-reforming or humidifying a fuel, when methane and other gases are used as the fuel.
2. Description of the Related Art
Heretofore, fuel cells have been developed and commercially implemented as low-pollution power generating means to replace traditional power generating means such as thermal power generation, or as electric energy sources for electric vehicles that replace traditional engine-driven vehicles using gasoline or the like as a fuel. Especially, in recent years, much research work has been done for the development of higher-efficiency, higher-performance, and lower-cost fuel cells.
As is well known, there are various types of fuel cells, distinguished by the method of power generation. Among the well-known types of fuel cells, the type of fuel cell that uses a solid electrolyte, that is, the solid oxide fuel cell (SOFC), is attracting attention in various technical fields because of its potential to achieve the highest power generation efficiency and because the life can be extended and the cost reduced. In one example of such a solid oxide fuel cell, a calcined body formed from yttria(Y2O3)-doped stabilized zirconia is used as an oxygen ion conducting solid electrolyte layer. This fuel cell comprises an air electrode (cathode layer) formed on one side of the solid electrolyte layer and a fuel electrode (anode layer) on the opposite side thereof. The fuel cell comprising the solid electrolyte layer, the anode layer, and the cathode layer is housed in a chamber to complete a fuel cell battery. Power can be generated by supplying an oxygen or oxygen-containing gas to the cathode layer side of the fuel cell and a fuel gas such as methane to the anode layer side of the fuel cell. In this fuel cell battery, the oxygen (O2) supplied to the cathode layer is converted into oxygen ions (O2−) at the boundary between the cathode layer and the solid electrolyte layer, and the oxygen ions are conducted through the solid electrolyte layer into the anode layer where the ions react with the fuel gas, for example, a methane gas (CH4), supplied to the anode layer, producing water (H2O) and carbon dioxide (CO2) as final products. In this reaction process, a potential difference occurs between the cathode layer and the anode layer. Here, when the cathode layer and the anode layer are electrically connected by a lead wire, the electrons in the anode layer flow toward the cathode layer via the lead wire, and the fuel cell thus generates electrical power.
Various improvements have been made in the above type of fuel cell and in other types of fuel cell in order to increase power generating efficiency, etc. For example, Japanese Unexamined Patent Publication (Kokai) No. 5-255796 describes a nickel cermet that can be advantageously used as a fuel electrode and, in particular, in a solid oxide fuel cell, and a method of manufacturing the same. The nickel cermet described in this patent document consists essentially of 35 to 70% by weight of a metal nickel phase and 65 to 30% by weight of a zirconia phase stabilized in the cubic form with yttria, and the two phases are distinctly and homogeneously distributed at a level lower than 1 μm, the dispersion of nickel in percentage being 0.2 to 2.0 and the specific surface area being 2 to 12 m2/g (nickel) and 1 to 4 m2/g (cermet).
Fuel cells using a nickel cermet as a fuel electrode are also proposed in recent years. For example, Japanese Unexamined Patent Publication (Kokai) No. 2004-127761 describes a fuel electrode for a solid oxide fuel cell wherein the fuel electrode is formed by compounding mother particles of metal oxides such as NiO (nickel oxide), CoO (cobalt oxide), etc. with child particles of oxygen ion conducting ceramic materials such as YSZ (yttria-stabilized zirconia), PSZ (partially stabilized zirconia), etc. and by calcining the resulting composite powder.
On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 2005-19261 describes a fuel electrode for a solid oxide fuel cell wherein the fuel electrode is formed by calcining a powder mixture prepared by mixing fine zirconia powders whose 50 percent has a particle size within the range of 0.4 to 0.8 μm, coarse zirconia powders whose 50 percent has a particle size within the range of 25 to 50 μm, and nickel oxide powders whose 50 percent has a particle size of larger than 2 μm but smaller than 5 μm.
However, fuel cells using a nickel cermet as a fuel electrode have unsolved problems. For example, when a methane gas is used as the fuel, if the fuel electrode is formed from a nickel cermet, there arises not only the problem that high fuel electrode performance cannot be achieved because the activity of the fuel electrode is relatively low, but also the problem that carbon precipitates on the surface of the fuel electrode. Further, in fuel cells, usually, a noble metal such as platinum is used as a catalyst in order to enhance performance. However, as platinum, for example, is a limited resource and is expensive, it is desired to develop a fuel electrode that does not use such a noble metal catalyst.